FIG. 1 is a simplified explanatory view showing an example of a conventional process 1 for manufacturing a seamless pipe such as a seamless steel pipe. In this process 1, a rod-shaped billet is pierced in a piercing mill (both not shown) to form a rough pipe (hollow shell) 4.
The hollow shell 4 undergoes elongation rolling using a mandrel mill 2 which has rolling stands 2a–2c equipped with caliber rolls and which reduces the wall thickness of the hollow shell 4 between the caliber rolls and a mandrel bar 5. Sizing is then performed using a sizing mill 3 having rolling stands 3a–3c equipped with three caliber rolls installed at equal intervals of 120° in the circumferential direction. In this manner, a seamless pipe having a prescribed outer diameter and wall thickness is manufactured.
The seamless pipe which has undergone sizing has thickness variations where its wall thickness locally varies in the circumferential direction of the pipe. There is a prescribed standard for the allowable extent of the thickness variation in a product. Up to the present time, in order to satisfy the standard, in the mandrel mill 2, thickness variations caused only by elongation rolling in the mandrel mill 2 were suppressed, and in the sizing mill 3, thickness variations caused only by sizing in the sizing mill 3 were suppressed. Namely, in the past, elongation rolling of hollow shell 4 was carried out so that thickness variations did not occur at the completion of elongation rolling. The resulting rough pipe (mother tube) 4 was placed into a reheating furnace 6, and after heating to a uniform temperature so as not to produce thickness variations during sizing, sizing was carried out with a sizing mill 3 (see the heating steps shown by dashed arrows in FIG. 1).
In recent years, with the object of improving productivity, as shown by the solid arrows in FIG. 1, sizing has come to be carried out by a sizing mill 3 on a mother tube 4 which has undergone elongation rolling in a mandrel mill 2 immediately after the completion of elongation rolling without performing heating in a reheating furnace 6. However, if heating in a reheating furnace 6 is not performed, the temperature distribution in the circumferential direction of the mother tube 4 which is introduced into the sizing mill 3 becomes nonuniform for the following reasons (a)–(c).
(a) The portion of the mother tube 4 which is reduced by the last rolling stand 2c of the mandrel mill 2 is transported from the mandrel mill 2 with the mandrel bar 5 still inserted into the interior of the mother tube 4, and then the mandrel bar 5 is pulled out of the mother tube 4. During this period, the heat of the mother tube 4 is transferred to the mandrel bar 5, so the temperature of the portion of the mother tube 4 which is reduced in the last stand 2c is lower than the temperature of other portions of the mother tube 4. The decrease in temperature increases as the length of time from when the elongation rolling by the mandrel mill 2 is completed until when the mandrel bar 5 is pulled out of the mother tube 4 increases.
(b) As shown in FIG. 1, with an ordinary two-roll mandrel mill, the pairs of caliber rolls in each rolling stand 2a–2c are arranged in series with the reduction direction varying by 90° between each pair. With this arrangement, at the portions of the mother tube 4 located at 45°, measured from the axis of the mother tube 4, with respect to the direction of reduction of the caliber rollers, the outer surface of the mother tube 4 contacts the caliber rolls in each stand and the corresponding inner surface contacts the mandrel bar 5. Therefore, the decrease in temperature of the outer and inner surfaces of these portions of the mother tube 4 located at 45° with respect to the direction of reduction becomes markedly greater than the decrease in the temperature of the outer and inner surface of other portions of the mother tube.
(c) When the number of even numbered rolling stands of the mandrel mill 2 (rolling stand 2b in the illustrated example) is different from the number of odd numbered rolling stands (rolling stands 2a and 2c in the illustrated example) or when the reduction which is carried out is not the same for each of rolling stands 2a–2c, a temperature difference develops in the mother tube 4 in the direction of reduction.
In the sizing mill 3, since a reduction in the outer diameter of the mother tube 4 is produced without using a mandrel bar to restrain the inner surface of the mother tube 4, the wall thickness of the mother tube 4 typically increases during sizing. In particular, portions of the mother tube 4 having a high temperature undergo a larger increase in wall thickness than portions at a low temperature due to having a lower resistance to deformation. Therefore, variations in thickness in which the wall thickness locally varies in the circumferential direction are produced in a seamless pipe during sizing. As a result, at the completion of sizing, the wall thickness of portions which contact the caliber rolls of the last rolling stand 2c of the mandrel mill 2 and the wall thickness of portions spaced from the direction of reduction by 45° are thinner than the wall thickness of other portions.
Japanese Published Unexamined Patent Application Hei 1-284411 (referred to below as Patent Document 1) discloses an invention in which thickness variations caused by elongation rolling of a seamless pipe are suppressed by forming grooves in the surface of the caliber rolls of a mandrel mill in order to cancel local decreases in thickness.